1. Field of the Invention
The application relates to a method and a related communication device used in a wireless communication system, and more particularly, to a method of handling an emergency PDN connection in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple user equipments (UEs), and communicates with a core network including a mobility management entity (MME), serving gateway, etc for Non Access Stratum (NAS) control. In UMTS system, a Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets to the mobile devices back and forth within its geographical service area, including packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN.
An access point name (APN) is used in the LTE system and the LTE-A system for establishing packet data connections, and is managed by the MME. Further, the APN is used for identifying a packet data network (PDN) or a service type (e.g. multimedia messaging service (MMS)), wherein the PDN or the service type is carried on the packet data connections.
The core network manages the mobile reachability by using mobile reachable timer, paging proceed factor (PPF), and implicit detach timer. The core network supervises the periodic tracking/routing/location area updating procedure of the UE by means of the mobile reachable timer. By default, the mobile reachable timer is set slightly larger than the periodic TAU/RAU/LAU timer. Upon expiry of the mobile reachable timer the network shall clear the PPF flag and start the implicit detach timer. The value of the implicit detach timer is network dependent. Due to the clear of the PPF, the core network cannot page the UE during the period of the implicit detach timer. If the implicit detach timer expires before the UE contacts the network, the network shall implicitly detach the UE. The implicit detach timer shall be stopped when a NAS signaling connection is established for the UE.
NAS level congestion control contains the functions: “APN based congestion control” and “General NAS level Mobility Management control”. The use of the APN based congestion control is for avoiding and handling of EMM and ESM signalling congestion associated with UEs with a particular APN. With General NAS level Mobility Management control, the MME may also use the reject of NAS level Mobility Management signaling requests under general congestion conditions.
When the NAS level mobility management congestion control is applied due to network overload or congestion, the MME rejects an attach request, a tracking area update (TAU), a routing area update (RAU) or a service request, and the MME replies the UE with a Mobility Management back-off timer.
When congestion control is active, the MME/SGSN stores the Mobility Management back-off timer and the Session Management back-off timer. Then the MME/SGSN enforces the stored back-off time by immediately rejecting any subsequent request from the UE targeting at connecting to the APN before the stored back-off time is expired.
However, the UE is allowed to initiate a Mobility Management procedure or a Session Management procedure with the APN for a high priority service or an emergency service, even when the Mobility Management back-off timer or the Session Management back-off timer is running. In this situation, since the APN is still in the congestion and has no resource for providing the high priority service or the emergency service, the MME rejects the high priority service or the emergency service, and a delay to access the high priority service or the emergency service is caused. Therefore, disaster may happen due to the delay of the high priority service or the emergency service.
When MME/SGSN performs NAS level congestion control, the MME/SGSN stores a Mobility Management back-off time and a Session Management back-off time. Then the MME/SGSN enforces the stored back-off time by immediately rejecting any subsequent MM/SM request from the UE and allocating a MM/SM back-off timer. However the MME/SGSN does not stores allocated MM/SM back-off timer in the EMM/ESM (EPS Mobility Management, EMM/EPS Session Management, ESM) context per UE basis.
For a normal attached UE, if it gets rejection with a MM back-off timer due to TAU reject or service reject. The UE is normal attached (EMM-Registered state) but enters ECM-IDLE mode when the MM back-off timer is running. When MM back-off timer is running, the back-off UE is still allowed to make emergency call by PDN connection request with request type of emergency to an emergency APN. When the NAS signaling connection is built due to emergency PDN connection request and the UE returns from ECM-IDLE mode to ECM-Connected mode, the network stops the mobile reachable timer.
Considering a UE is rejected by NAS MM requests message with a MM back-off (MMBK) timer, the back-off UE establishes an emergency PDN connections while the MM back-off timer is running. When the back-off UE with non-emergency PDN connection and emergency PDN connection in connected mode moves to a new Tracking area, before handover procedure the back-off UE requests TAU if it enters a new tracking area that is not in the TA list. Since the mobile reachable timer (MRT) is not stored in the MM context and sent to the target MME/SGSN from the source MME/SGSN, the target MME/SGSN does not know how to handle the reachability of the back-off UE and may implicitly detach the back-off UE when the MRT is expired.